Chuck for holding a device under test

ABSTRACT

A chuck for a probe station.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. Pat. App. No. 11/204,910, filedAug. 15, 2005, now U.S. Pat. No. 7,352,168; which is a continuation ofU.S. Pat. App. No. 09/877,823, filed Jun. 7, 2001, now U.S. Pat. No.6,965,226, which claims the benefit of U.S. Provisional App. No.60/230,212, filed Sep. 5, 2000.

BACKGROUND OF THE INVENTION

The present application relates to an improved chuck.

With reference to FIGS. 1, 2 and 3, a probe station comprises a base 10(shown partially) which supports a platen 12 through a number of jacks14 a, 14 b, 14 c, 14 d which selectively raise and lower the platenvertically relative to the base by a small increment (approximatelyone-tenth of an inch) for purposes to be described hereafter. Alsosupported by the base 10 of the probe station is a motorized positioner16 having a rectangular plunger 18 which supports a movable chuckassembly 20 for supporting a wafer or other test device. The chuckassembly 20 passes freely through a large aperture 22 in the platen 12which permits the chuck assembly to be moved independently of the platenby the positioner 16 along X, Y and Z axes, i.e., horizontally along twomutually-perpendicular axes X and Y, and vertically along the Z axis.Likewise, the platen 12, when moved vertically by the jacks 14, movesindependently of the chuck assembly 20 and the positioner 16.

Mounted atop the platen 12 are multiple individual probe positionerssuch as 24 (only one of which is shown), each having an extending member26 to which is mounted a probe holder 28 which in turn supports arespective probe 30 for contacting wafers and other test devices mountedatop the chuck assembly 20. The probe positioner 24 has micrometeradjustments 34, 36 and 38 for adjusting the position of the probe holder28, and thus the probe 30, along the X, Y and Z axes, respectively,relative to the chuck assembly 20. The Z axis is exemplary of what isreferred to herein loosely as the “axis of approach” between the probeholder 28 and the chuck assembly 20, although directions of approachwhich are neither vertical nor linear, along which the probe tip andwafer or other test device are brought into contact with each other, arealso intended to be included within the meaning of the term “axis ofapproach.” A further micrometer adjustment 40 adjustably tilts the probeholder 28 to adjust planarity of the probe with respect to the wafer orother test device supported by the chuck assembly 20. As many as twelveindividual probe positioners 24, each supporting a respective probe, maybe arranged on the platen 12 around the chuck assembly 20 so as toconverge radially toward the chuck assembly similarly to the spokes of awheel. With such an arrangement, each individual positioner 24 canindependently adjust its respective probe in the X, Y and Z directions,while the jacks 14 can be actuated to raise or lower the platen 12 andthus all of the positioners 24 and their respective probes in unison.

An environment control enclosure is composed of an upper box portion 42rigidly attached to the platen 12, and a lower box portion 44 rigidlyattached to the base 10. Both portions are made of steel or othersuitable electrically conductive material to provide EMI shielding. Toaccommodate the small vertical movement between the two box portions 42and 44 when the jacks 14 are actuated to raise or lower the platen 12,an electrically conductive resilient foam gasket 46, preferably composedof silver or carbon-impregnated silicone, is interposed peripherally attheir mating juncture at the front of the enclosure and between thelower portion 44 and the platen 12 so that an EMI, substantiallyhermetic, and light seal are all maintained despite relative verticalmovement between the two box portions 42 and 44. Even though the upperbox portion 42 is rigidly attached to the platen 12, a similar gasket 47is preferably interposed between the portion 42 and the top of theplaten to maximize sealing.

With reference to FIGS. 5A and 5B, the top of the upper box portion 42comprises an octagonal steel box 48 having eight side panels such as 49a and 49 b through which the extending members 26 of the respectiveprobe positioners 24 can penetrate movably. Each panel comprises ahollow housing in which a respective sheet 50 of resilient foam, whichmay be similar to the above-identified gasket material, is placed. Slitssuch as 52 are partially cut vertically in the foam in alignment withslots 54 formed in the inner and outer surfaces of each panel housing,through which a respective extending member 26 of a respective probepositioner 24 can pass movably. The slitted foam permits X, Y and Zmovement of the extending members 26 of each probe positioner, whilemaintaining the EMI, substantially hermetic, and light seal provided bythe enclosure. In four of the panels, to enable a greater range of X andY movement, the foam sheet 50 is sandwiched between a pair of steelplates 55 having slots 54 therein, such plates being slidabletransversely within the panel housing through a range of movementencompassed by larger slots 56 in the inner and outer surfaces of thepanel housing.

Atop the octagonal box 48, a circular viewing aperture 58 is provided,having a recessed circular transparent sealing window 60 therein. Abracket 62 holds an apertured sliding shutter 64 to selectively permitor prevent the passage of light through the window. A stereoscope (notshown) connected to a CRT monitor can be placed above the window toprovide a magnified display of the wafer or other test device and theprobe tip for proper probe placement during set-up or operation.Alternatively, the window 60 can be removed and a microscope lens (notshown) surrounded by a foam gasket can be inserted through the viewingaperture 58 with the foam providing EMI, hermetic and light sealing. Theupper box portion 42 of the environment control enclosure also includesa hinged steel door 68 which pivots outwardly about the pivot axis of ahinge 70 as shown in FIG. 2A. The hinge biases the door downwardlytoward the top of the upper box portion 42 so that it forms a tight,overlapping, sliding peripheral seal 68 a with the top of the upper boxportion. When the door is open, and the chuck assembly 20 is moved bythe positioner 16 beneath the door opening as shown in FIG. 2A, thechuck assembly is accessible for loading and unloading.

With reference to FIGS. 3 and 4, the sealing integrity of the enclosureis likewise maintained throughout positioning movements by the motorizedpositioner 16 due to the provision of a series of four sealing plates72, 74, 76 and 78 stacked slidably atop one another. The sizes of theplates progress increasingly from the top to the bottom one, as do therespective sizes of the central apertures 72 a, 74 a, 76 a and 78 aformed in the respective plates 72, 74, 76 and 78, and the aperture 79 aformed in the bottom 44 a of the lower box portion 44. The centralaperture 72 a in the top plate 72 mates closely around the bearinghousing 18 a of the vertically-movable plunger 18. The next plate in thedownward progression, plate 74, has an upwardly-projecting peripheralmargin 74 b which limits the extent to which the plate 72 can slideacross the top of the plate 74. The central aperture 74 a in the plate74 is of a size to permit the positioner 16 to move the plunger 18 andits bearing housing 18 a transversely along the X and Y axes until theedge of the top plate 72 abuts against the margin 74 b of the plate 74.The size of the aperture 74 a is, however, too small to be uncovered bythe top plate 72 when such abutment occurs, and therefore a seal ismaintained between the plates 72 and 74 regardless of the movement ofthe plunger 18 and its bearing housing along the X and Y axes. Furthermovement of the plunger 18 and bearing housing in the direction ofabutment of the plate 72 with the margin 74 b results in the sliding ofthe plate 74 toward the peripheral margin 76 b of the next underlyingplate 76. Again, the central aperture 76 a in the plate 76 is largeenough to permit abutment of the plate 74 with the margin 76 b, butsmall enough to prevent the plate 74 from uncovering the aperture 76 a,thereby likewise maintaining the seal between the plates 74 and 76.Still further movement of the plunger 18 and bearing housing in the samedirection causes similar sliding of the plates 76 and 78 relative totheir underlying plates into abutment with the margin 78 b and the sideof the box portion 44, respectively, without the apertures 78 a and 79 abecoming uncovered. This combination of sliding plates and centralapertures of progressively increasing size permits a full range ofmovement of the plunger 18 along the X and Y axes by the positioner 16,while maintaining the enclosure in a sealed condition despite suchpositioning movement. The EMI sealing provided by this structure iseffective even with respect to the electric motors of the positioner 16,since they are located below the sliding plates.

With particular reference to FIGS. 3, 6 and 7, the chuck assembly 20 isa modular construction usable either with or without an environmentcontrol enclosure. The plunger 18 supports an adjustment plate 79 whichin turn supports first, second and third chuck assembly elements 80, 81and 83, respectively, positioned at progressively greater distances fromthe probe(s) along the axis of approach. Element 83 is a conductiverectangular stage or shield 83 which detachably mounts conductiveelements 80 and 81 of circular shape. The element 80 has a planarupwardly-facing wafer-supporting surface 82 having an array of verticalapertures 84 therein. These apertures communicate with respectivechambers separated by O-rings 88, the chambers in turn being connectedseparately to different vacuum lines 90 a, 90 b, 90 c (FIG. 6)communicating through separately-controlled vacuum valves (not shown)with a source of vacuum. The respective vacuum lines selectively connectthe respective chambers and their apertures to the source of vacuum tohold the wafer, or alternatively isolate the apertures from the sourceof vacuum to release the wafer, in a conventional manner. The separateoperability of the respective chambers and their corresponding aperturesenables the chuck to hold wafers of different diameters.

In addition to the circular elements 80 and 81, auxiliary chucks such as92 and 94 are detachably mounted on the corners of the element 83 byscrews (not shown) independently of the elements 80 and 81 for thepurpose of supporting contact substrates and calibration substrateswhile a wafer or other test device is simultaneously supported by theelement 80. Each auxiliary chuck 92, 94 has its own separateupwardly-facing planar surface 100, 102 respectively, in parallelrelationship to the surface 82 of the element 80. Vacuum apertures 104protrude through the surfaces 100 and 102 from communication withrespective chambers within the body of each auxiliary chuck. Each ofthese chambers in turn communicates through a separate vacuum line and aseparate independently-actuated vacuum valve (not shown) with a sourceof vacuum, each such valve selectively connecting or isolating therespective sets of apertures 104 with respect to the source of vacuumindependently of the operation of the apertures 84 of the element 80, soas to selectively hold or release a contact substrate or calibrationsubstrate located on the respective surfaces 100 and 102 independentlyof the wafer or other test device. An optional metal shield 106 mayprotrude upwardly from the edges of the element 83 to surround the otherelements 80, 81 and the auxiliary chucks 92, 94.

All of the chuck assembly elements 80, 81 and 83, as well as theadditional chuck assembly element 79, are electrically insulated fromone another even though they are constructed of electrically conductivemetal and interconnected detachably by metallic screws such as 96. Withreference to FIGS. 3 and 3A, the electrical insulation results from thefact that, in addition to the resilient dielectric O-rings 88,dielectric spacers 85 and dielectric washers 86 are provided. These,coupled with the fact that the screws 96 pass through oversizedapertures in the lower one of the two elements which each screw joinstogether thereby preventing electrical contact between the shank of thescrew and the lower element, provide the desired insulation. As isapparent in FIG. 3, the dielectric spacers 85 extend over only minorportions of the opposing surface areas of the interconnected chuckassembly elements, thereby leaving air gaps between the opposingsurfaces over major portions of their respective areas. Such air gapsminimize the dielectric constant in the spaces between the respectivechuck assembly elements, thereby correspondingly minimizing thecapacitance between them and the ability for electrical current to leakfrom one element to another. Preferably the spacers and washers 85 and86, respectively, are constructed of a material having the lowestpossible dielectric constant consistent with high dimensional stabilityand high volume resistivity. A suitable material for the spacers andwashers is glass epoxy, or acetyl homopolymer marketed under thetrademark Delrin by E. I. DuPont.

With reference to FIGS. 6 and 7, the chuck assembly 20 also includes apair of detachable electrical connector assemblies designated generallyas 108 and 110, each having at least two conductive connector elements108 a, 108 b and 110 a, 110 b, respectively, electrically insulated fromeach other, with the connector elements 108 b and 110 b preferablycoaxially surrounding the connector elements 108 a and 110 a as guardstherefor. If desired, the connector assemblies 108 and 110 can betriaxial in configuration so as to include respective outer shields 108c, 110 c surrounding the respective connector elements 108 b and 110 b,as shown in FIG. 7. The outer shields 108 c and 110 c may, if desired,be connected electrically through a shielding box 112 and a connectorsupporting bracket 113 to the chuck assembly element 83, although suchelectrical connection is optional particularly in view of thesurrounding EMI shielding enclosure 42, 44. In any case, the respectiveconnector elements 108 a and 110 a are electrically connected inparallel to a connector plate 114 matingly and detachably connectedalong a curved contact surface 114 a by screws 114 b and 114 c to thecurved edge of the chuck assembly element 80. Conversely, the connectorelements 108 b and 110 b are connected in parallel to a connector plate116 similarly matingly connected detachably to element 81. The connectorelements pass freely through a rectangular opening 112 a in the box 112,being electrically insulated from the box 112 and therefore from theelement 83, as well as being electrically insulated from each other. Setscrews such as 118 detachably fasten the connector elements to therespective connector plates 114 and 116.

Either coaxial or, as shown, triaxial cables 118 and 120 form portionsof the respective detachable electrical connector assemblies 108 and110, as do their respective triaxial detachable connectors 122 and 124which penetrate a wall of the lower portion 44 of the environmentcontrol enclosure so that the outer shields of the triaxial connectors122, 124 are electrically connected to the enclosure. Further triaxialcables 122 a, 124 a are detachably connectable to the connectors 122 and124 from suitable test equipment such as a Hewlett-Packard 4142B modularDC source/monitor or a Hewlett-Packard 4284A precision LCR meter,depending upon the test application. If the cables 118 and 120 aremerely coaxial cables or other types of cables having only twoconductors, one conductor interconnects the inner (signal) connectorelement of a respective connector 122 or 124 with a respective connectorelement 108 a or 110 a, while the other conductor connects theintermediate (guard) connector element of a respective connector 122 or124 with a respective connector element 108 b, 110 b. U.S. Pat. No.5,532,609 discloses a probe station and chuck and is hereby incorporatedby reference.

The chuck assembly 20 with corresponding vertical apertures 84 andrespective chambers separated by O-rings 88 permits selectively creatinga vacuum within three different zones. Including the three O-rings 88and the dielectric spacers 85 surrounding the metallic screws 96 permitssecuring adjacent first, second and third chuck assembly elements 80, 81and 83 together. The concentric O-rings 88 are squeezed by the first andsecond chuck assembly elements and assist in distributing the forceacross the upper surface of the chuck assembly 20 to maintain a flatsurface. However, the O-rings and dielectric spacers 85 have a greaterdielectric constant than the surrounding air resulting in leakagecurrents. Also, the additional material between adjoining chuck assemblyelements 80, 81, and 83 decreases the capacitance between the adjoiningchuck assembly elements. Moreover, the dielectric material of theO-rings and dielectric spacers 85 builds up a charge therein duringtesting which increases the dielectric absorption. The O-rings anddielectric spacers 85 provides mechanical stability against warping thechuck when a wafer thereon is probed so that thinner chuck assemblyelements 80, 81, and 83 may be used. The height of the different O-ringsand dielectric spacers 85 tend to be slightly different which introducesnon-planarity in the upper surface when the first, second, and thirdchuck assembly elements 80, 81, and 83 are secured together.

The foregoing and other objectives, features, and advantages of theinvention will be more readily understood upon consideration of thefollowing detailed description of the invention, taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a partial front view of an exemplary embodiment of a waferprobe station constructed in accordance with the present invention.

FIG. 2 is a top view of the wafer probe station of FIG. 1.

FIG. 2A is a partial top view of the wafer probe station of FIG. 1 withthe enclosure door shown partially open.

FIG. 3 is a partially sectional and partially schematic front view ofthe probe station of FIG. 1.

FIG. 3A is an enlarged sectional view taken along line 3A-3A of FIG. 3.

FIG. 4 is a top view of the sealing assembly where the motorizedpositioning mechanism extends through the bottom of the enclosure.

FIG. 5A is an enlarged top detail view taken along line 5A-5A of FIG. 1.

FIG. 5B is an enlarged top sectional view taken along line 5B-5B of FIG.1.

FIG. 6 is a partially schematic top detail view of the chuck assembly,taken along line 6-6 of FIG. 3.

FIG. 7 is a partially sectional front view of the chuck assembly of FIG.6.

FIG. 8 is a perspective view of a chuck illustrating a set of spacersand vacuum interconnections.

FIG. 9 is a plan view of the bottom surface of the upper chuck assemblyelement.

FIG. 10 is a plan view of the upper surface of the upper chuck assemblyelement.

FIG. 11 is a cross sectional view of a multi-layer chuck.

FIG. 12 is an enlarged cross sectional view of the interconnectionbetween a pair of chuck assembly elements of the chuck of FIG. 11.

FIG. 13 is an enlarged cross sectional view of the interconnectionbetween a pair of chuck assembly elements of the chuck of FIG. 11illustrating a minimum air breakdown distance.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Traditionally chuck designers use thin chuck assembly elements and manyradially arranged screws in order to permit the screws to be tightenedtightly without significantly warping any of the chuck assemblyelements, and in particular the upper chuck assembly element.Maintaining a flat planar upper chuck assembly element is important topermit accurate probing of the wafer and avoid breaking, or otherwisedamaging, the wafer while probing. In a multi-layered chuck, the lowerchuck assembly element is secured to the middle chuck assembly element,the middle chuck assembly element in turn is secured to the upper chuckassembly element, which results in any non-uniformities of slightlydifferent thicknesses of the chuck assembly elements and interposeddielectric elements creating a cumulative non-planarity. For example,non-uniformity in the planarity of the lower chuck assembly element anddifferences in the thickness of the dielectric spacers may result in themiddle chuck assembly element being slightly warped when securedthereto. Non-uniformity in the planarity of the middle chuck assemblyelement, the slight warping of the middle chuck assembly element, andthe differences in the thickness of the dielectric spacers and O-rings,may result significant warping of the upper chuck assembly element whensecured to the middle chuck assembly element. Accordingly, thethicknesses and planarity of (1) each chuck assembly element, (2)dielectric spacers, and (3) O-rings, needs to be accurately controlledin order to achieve a planar upper surface of the upper chuck assemblyelement.

After consideration of the thin chuck assembly elements and the desireto minimize warping of the upper chuck assembly element, the presentinventor came to the realization that a three point securement system,including for example three pins, permits defining the orientation ofthe upper chuck assembly element without inducing stress into the upperchuck assembly element 180, as illustrated in FIG. 8. Preferably, thepins are substantially equal distant from one another. Changes in thespacing of the height of any of the pins 200, 202, 204 results inpivoting the upper chuck assembly element 180 about the remaining twopins in a manner free from introducing added stress and hencenon-planarity of the upper surface 198 of the upper chuck assemblyelement. There are preferably no dielectric spacers which maintain, orotherwise define, the spacing between the upper and middle chuckassembly elements, other than the pins 200, 202, 204. The elimination ofdielectric spacers, such as O-rings, avoids stressing the upper chuckassembly element when under pressing engagement with the middle chuckassembly element. Another benefit that may be achieved by using a threepoint system is that the orientation of the upper surface of the upperchuck assembly element may be defined with respect to the prober stageand probes with minimal, if any, planarization of the interveninglayers. In other words, if the planarity of the middle and lower chuckassembly elements is not accurately controlled, the planarity of theupper chuck assembly element will not be affected. Normally the spacingbetween the upper/middle and middle/lower chuck assembly elements isrelatively uniform to provide relatively uniform capacitance between therespective chuck assembly elements. It is to be understood that anysuitable interconnection assembly involving three discrete points orregions of the chuck assembly elements may be employed.

Minimization of the spacers, such as O-rings, between the upper andmiddle chuck assembly elements reduces the capacitive coupling betweenthe upper and middle chuck assembly elements to less than it would havebeen with additional dielectric layer material there between. Theelimination of additional spacers likewise increases the resistancebetween adjacent chuck assembly elements.

Connecting each vacuum line(s) directly to the center of the upper chuckassembly element 180 normally requires at least one corresponding holedrilled radially into the upper chuck assembly element from whichvertically extending vacuum chambers provide a vacuum to the uppersurface 198 of the upper chuck assembly element. Machining thecombination of radial and vertical holes requires highly accuratemachining which is difficult, time consuming, and expensive. Machiningsuch holes becomes increasingly more difficult as the size of the chucksincreases.

After consideration of the difficulty of machining accurate holes intothe side of the upper chuck assembly element 180, the present inventordetermined that machining a set of airways 210 a-210 e in the lowersurface 208 of the upper chuck assembly element is easier and tends tobe more accurate, as shown in FIG. 9. In addition, the airways 210 a-210e in the lower surface 208 of the chuck may be readily cleaned of dustand debris. The lower surface 208 of the upper chuck assembly element iscovered with a cover plate 212 (see FIG. 11), which is preferably thin.The cover plate 212 is preferably secured to the upper chuck assemblywith glue (not shown) and a thin layer of vacuum grease to provide aseal there between. Preferably, the cover plate 212 is conductivematerial electrically connected to the upper chuck assembly element. Itis to be understood that the cover plate may be made of any materialhaving any thickness, as desired. Referring to FIG. 10, a plurality of“zones” defined by vacuum holes 214 a-214 e to the upper surface 198 maybe achieved, each of which is preferably concentric in nature, so thateach “zone” may be individually controlled and provided a vacuum, ifdesired. This provides accurate pressure control for different sizes ofwafers. For example, the diameters of the concentric rings may be, 2½″,5½″, 7½″, and 11½″ to accommodate wafers having sizes of 3″, 6″, 8″, and12″. This permits the system to be selectively controlled to accommodatethe size of the wafer being tested so that uncovered vacuum holes arenot attempting to provide a vacuum, which may reduce the vacuum pressureavailable and pull contaminated air through the system. Dust and otherdebris in contaminated air may result in a thin layer of dust within thevacuum interconnections, described later, resulting in a decrease inelectrical isolation between the upper and middle chuck assemblyelements. It is to be understood that any suitable structure may be usedto define a series of airways between adjacent layers of material, suchmaterials preferably being conductive and in face-to-face engagement.The definition of airways may even be used with chucks where the vacuumlines are interconnected to the upper chuck assembly element, togetherwith the definition of airway.

The elimination of the O-rings between the adjacent upper and middlechuck assembly elements creates a dilemma as to of how to provide avacuum to the top surface of the upper chuck assembly element, ifdesired. The present inventor determined that it is normally undesirableto attach a vacuum tube directly to the upper chuck assembly elementbecause the exterior conductive surface of the vacuum tube is normallyconnected to shield potential. The shield potential of the exterior ofthe vacuum tube directly adjoining the upper chuck assembly elementwould result in an unguarded leakage current between the upper chuckassembly and the vacuum tube.

To provide a vacuum path between the middle chuck assembly element andthe upper chuck assembly element a vacuum pin 206 interconnectsrespective vacuum lines and particular vacuum holes (e.g., “zones”) onthe upper surface of the upper chuck assembly element, as illustrated inFIG. 11. Normally, one vacuum line and one vacuum pin is provided foreach “zone.” The vacuum pins are preferably recessed into respectiveopenings 220 a and 220 b in the facing surfaces 208 and 224 of the upperand middle chuck assembly elements. Each vacuum pin includes a pair ofO-rings 222 a and 222 b which provides a seal within respective openings220 a and 220 b and likewise permits the vacuum pins 206 to move withinthe openings. The spacing between the facing surfaces 208 and 224, depthof the openings 220 a and 220 b, and length of the vacuum pins 206 arepreferably selected such that changes in the spacing between thesurfaces still permit the vacuum pins 206 some movement within theopenings 220 a and 220 b. Accordingly, the vacuum pins “float” withinthe openings and do not determine, or otherwise limit, the spacingbetween the upper and middle chuck assembly elements. Further, thevacuum pins are not rigidly connected to both the upper and middle chuckassembly elements. Alternatively, the vacuum pins may be rigidlyconnected to one of the upper and middle chuck assembly elements, ifdesired. The vacuum pins are preferably constructed from a gooddielectric material, such as Teflon or PCTFE. Preferably, the vacuumpin(s) are positioned at locations exterior to the pins 200, 201, 204(e.g., the distance from the center of the chuck to the pins is lessthan the distance from the center of the chuck to the vacuum pins) tominimize noise. It is to be understood that any non-rigidlyinterconnected set (one or more) of vacuum paths that do not define thespacing may be provided between a pair of chuck assembly elements.

The pin securing the middle chuck assembly element 182 to the upperchuck assembly element 180 includes a portion thereunder that is open tothe lower chuck assembly element, normally connected to shield. Morespecifically, the pin 204 electrically connected to the upper chuckassembly element 180 provides an unguarded leakage path through themiddle chuck assembly element 182 to the lower chuck assembly element184. In existing designs, a small plate is secured over the opening toprovide guarding. A more convenient guarding structure is a lower coverplate 230 over the pin openings, preferably covering a major portion ofthe middle chuck assembly element 182. The lower cover plate 230 iselectrically isolated from the pins. In addition, the plate 230 togetherwith the middle chuck assembly element 182 defines vacuum paths.

Referring to FIG. 12, the pin structure provides both mechanicalstability and electrical isolation. A threaded screw 240 is insertedthrough the middle chuck assembly element 182 and threaded into athreaded opening 242 in the lower surface of the upper chuck assemblyelement 180. A conductive circular generally U-shaped member 244separates the upper and middle chuck assembly elements and is inpressing engagement with the upper chuck assembly element. Theconductive U-shaped member 244 is electrically connected to the screw240 and extends radially outward from the screw 240. The conductiveU-shaped member provides lateral stability of the chuck assembly. Aninsulating circular generally U-shaped member 246, preferably made fromPCTFE, opposes the conductive U-shaped member 244 and is in pressingengagement with the middle chuck assembly element. The insulatingcircular U-shaped member 246 self-centers to the conductive U-shapedmember 244 within the upwardly extending portions thereof. A circularinsulating insert 248 surrounds the threaded screw 240 within theopening 250 in the middle chuck assembly element and supports theinclined head portion 252 of the threaded screw 240. In the case thatthe screw 240 does not have an inclined portion the insulating insertmay support the head portion of the screw 240. An insulating cover 254is preferably placed over the end of the threaded screw 240 andpreferably spaced apart therefrom. Over the end of the screw is thecover plate 230, preferably connected to a guard potential. The pinstructure may likewise be used, if desired, between other adjacentplates of the chuck assembly.

While making high voltage measurements the air between two conductorswill break down, e.g., arc, if the conductors are sufficiently closetogether. For example, when testing at 5000 volts the spacing betweenconductors should be in excess of about 0.2 inches. Referring to FIG. 13(same as FIG. 12), it may be observed that all of the paths through theair from the screw and conductive circular U-shaped member (signalpotential) to another conductor at guard potential is greater than 0.2inches, as indicated by the “- - - ” lines. For example, the fins of theU-shaped insulating member 246 may increases the creepage distancegreater than about 0.2 inches.

After further consideration another factor impacting rigidly is theinterconnecting materials themselves. Preferably, the conductive memberis at least three times as thick as the insulating material between theadjacent chuck assembly elements, and more preferably at least six timesas thick. In this manner, a major portion of the spacing material isrigid conductive material which is significantly less prone tocompression than the insulating material under pressure.

After extensive testing the present inventor came to the furtherrealization that the dielectric absorption of the dielectric materialtends to drain faster when both sides of the dielectric material are inface-to-face contact with electrical conductors. In contrast, when onlyone side of the dielectric material is in face-to-face contact with anelectrical conductor then the dielectric absorption drains slowly withchanges in electrical potential and hence degrades the electricalperformance. Accordingly, referring to FIG. 12, it may be observed thatsubstantially all (or at least a major portion) of the insulatingmaterial in contact with a conductor has an opposing conductor. Forexample, the upper portion of the center insulating portion is not incontact with the conductive screw because it would be difficult toprovide an opposing conductor, and be further complicated if a requisitespacing is necessary.

The terms and expressions which have been employed in the foregoingspecification are used therein as terms of description and not oflimitation, and there is no intention, in the use of such terms andexpressions, of excluding equivalents of the features shown anddescribed or portions thereof, it being recognized that the scope of theinvention is defined and limited only by the claims which follow.

1. A chuck for a probe station comprising: (a) a first chuck assemblyelement having a lower surface and an upper surface thereon suitable tosupport a wafer; (b) a second chuck assembly element having an uppersurface and a lower surface where the upper surface is in opposingrelationship to said lower surface of said first chuck assembly element;(c) a chuck spacing mechanism interconnecting said first and secondchuck assembly elements and defining the spacing between said first andsecond chuck assembly elements; and (d) said chuck spacing mechanismincluding an insulator having a first surface in pressing engagementwith said upper surface of said second chuck assembly element and asecond surface spaced apart from said lower surface of said first chuckassembly and in pressing engagement with a first surface of a conductivespacer, a second surface of said conductive spacer in pressingengagement with said lower surface of said first chuck assembly element,where said first surface of said insulator in pressing engagement withsaid upper surface opposes said second surface and is substantiallycoextensive with said second surface of said insulator in pressingengagement with said conductive spacer.
 2. The chuck of claim 1 whereinsaid chuck spacing mechanism includes a central member extending therethrough.
 3. The chuck of claim 2 wherein said central member is rigidlyattachable to said first chuck assembly element.
 4. The chuck of claim 3wherein said central member is electrically isolated from said secondchuck assembly element.
 5. The chuck of claim 4 wherein said centralmember secures said first and second chuck assembly elements togetherwith said insulator and said conductive spacer defining said spacing.